The present invention relates to an amplitude modulated transmitter which employs a feedback loop which corrects for distortion. More particularly, the present invention relates to an amplitude modulated transmitter which uses a directional coupler in the feedback loop of a type which may be employed in the frequency range below 50 mega-hertz.
In the past, modulation had to be carried out in one of the last power amplifier stages in order to avoid excessive distortion while maintaining a high efficiency. This required a very large modulating signal which was usually applied to a vacuum tube modulator. Where low level modulation, that is, modulation at one of the early stages in the transmitter, was employed, all stages after the modulator had to employ relatively distortion-free amplifiers. This resulted in the use of amplifiers operated in class A mode or very close to class A. Amplifiers which are operated in class A mode are relatively inefficient, as contrasted to amplifiers which are operated in class C mode.
The present invention enables relatively distortion-free amplification of the modulated signal by monitoring the output of the transmitter, or the output of the output amplifier of the transmitter, even though there may be signals reflected from an antenna or other load which the transmitter is feeding. Examples of other loads may be additional amplification stages or the feeding of the output of the transmitter into a network such as the alternating current power system of a building.